Metabolic and molecular basis for the salt and alkali responses of Suaeda corniculata

Highlights

• Alkali stress largely influenced plant growth as compared to salt stress.

• Protein and metabolite profiles showed alkali and salt stress cause distinct molecular effect on S. corniculata.

• AsA-GSH cycle and Prx/Trx pathway mainly contributed to ROS removal upon stress condition.

• Stress induced proteins involved in fatty acid metabolism probably modulate alkali tolerance.

Abstract

High salinity and alkalinity, two of the most widespread abiotic stressors which largely impact plant growth and development. Most researches focus on how plant cope with neutral salt stress, the underlying mechanism of plant response to alkali stress has not been systematically studied, and little information is available as reference. The halophyte Suaeda corniculata is an outstanding pasture with strong tolerance to salt and alkali stress. Here, the combined analysis of ¹H NMR-based metabolomics, 2DE-based proteomics and physiological observation were performed, to demonstrate the comparison on metabolic and molecular network of S. corniculata responsive to salt and alkali stress. Our results showed that the alkali stress induced more severe effect on the plant growth than salt stress. The metabolomic and proteomic characteristics showed distinct regulation patterns in response to salt and alkali in S. corniculata. The evaluation of reactive oxygen species (ROS)-scavenging systems, as well as abundance patterns of salt and alkali-responsive proteins or metabolites indicated that the hyper activation of ROS scavenging system, including APX dependent AsA-GSH cycle and Prx/Trx pathway, played a major role in maintaining redox homeostasis to alleviate oxidative damage. In addition, the accumulation of glycine betaine mainly contributes to the re-establishment of osmotic homeostasis. Importantly, the high abundance of germin-like proteins and enzymes involved in fatty acid metabolism might positively contribute to the alkali tolerance of S. corniculata. Together, this study provides novel insights into the physiological and metabolic features in halophyte S. corniculata to salt and alkali stress, and this deepens our understanding of how halophytes reprogram the molecular activities to cope with salinity and alkalinity in soil.

Introduction

Saline-alkali soil is one of the major abiotic stresses limiting plant growth and crop productivity worldwide (Rana et al., 2008; Tavakkoli et al., 2011; Tuteja, 2007). Alkaline soil contains high levels of Na₂CO₃ and NaHCO₃, which leads to a high pH (>9.0) (Zhang et al., 2016). In northeastern China, alkalinized grasslands have reached more than 70% coverage due to the presence of excessive alkaline salts (Zhang et al., 2013). Compared to neutral salt, alkali salt induces high pH and imposes more severe damage to plants (Yang et al., 2007). A high pH environment surrounding plant roots has a great impact on nutrient uptake, organic acid balance, ion homeostasis, and especially pH stability in cells (Guo et al., 2010; Wang et al., 2009). Thus, it is urgently important to determine how plants respond to and tolerate alkali stress, which would help plants adapt to stressful environments. To date, plant salt tolerance has been extensively studied, but alkali tolerance is not understood.

Alkali stress largely affects the cellular metabolism of plants primarily due to the high pH shock and excess accumulation of inorganic ions attenuating protein synthesis in plant tissue (Hu et al., 2019). Even in the halophyte Helianthus annuus, which has high resistance to saline conditions, the protein concentration was drastically decreased upon Na₂CO₃ treatment (Manivannan et al., 2008). Nevertheless, several studies demonstrated that plants could modulate their metabolic function to overcome such an unfriendly environment. It has been revealed that an enhanced energy supply and the activation of multiple antioxidants play important roles in the Puccinellia tenuiflora response to alkali stress (Yu et al., 2013). In Na₂CO₃-treated seedlings of H. tuberosus, proteins involved in glycolysis, the TCA cycle, the PSI system, ROS scavenging and signal transduction were induced by alkali stress (Zhang et al., 2016). In addition, metabolic analysis in Malus halliana showed that the abundances of sucrose, amino acids, alkaloids, flavonoids and carotenoids were significantly upregulated, which contributes to improving the saline-alkali resistance of M. halliana by maintaining a balanced redox state (Jia et al., 2020).

Suaeda species are annual euhalophytes with highly succulent leaves that are able to accommodate salt, and they are important halophyte resources naturally distributed in saline and alkaline soils throughout the world (Flowers and Colmer, 2008). Based on an analysis under neutral salt stress, more than twenty species of Suaeda have been reported for their ability to survive in extremely saline soil, achieving benefits derived from the specific physiological and molecular regulation of Suaeda plants (Diao et al., 2021; Hasanuzzaman et al., 2014; Liu et al., 2018; Yuan et al., 2018). Most studies focused on S. salsa, and several salt tolerance genes were characterized, including aquaporins (AQPs) (Qi et al., 2009), vacuolar Na⁺/H⁺ antiporters (Qiu et al., 2007), V-ATPase and V-PPase (Wang et al., 2001), indicating that Suaeda plants could modulate water potential and ion distribution by succulent leaf to compartment excess Na⁺. Osmotic adjustment also plays a critical role in Suaeda adapting to high salt conditions, the accumulation of proline and soluble sugars could effectively promote osmoregulation (Behr et al., 2017). In addition, the overall increase in antioxidant system activity, including the enzymes SOD, POD, CAT, and GPX, and metabolites such as glutathione, ascorbate, flavonoids and phenylpropanoids, facilitates tolerance to salinity in Suaeda (Wu et al., 2011). Moreover, the fluctuation of plant hormones in Suaeda under salt stress may be involved in the regulation of salt tolerance (Guo et al., 2019).

S. corniculata, a member of the Suaeda genus, shows a higher alkali tolerance than S. salsa and can survive in saline-sodic soil with pH values ranging from 10 to 15.5 (Liu et al., 2011). However, the underlying mechanisms of the S. corniculata response to alkali stress are not well understood. Plant roots act as the primary site for perceiving salt signals, and their physiological and metabolic activities largely reflect the regulatory information dictating how plants adapt to stressful environments. Here, we investigated the comprehensive response of S. corniculata roots to NaCl and NaHCO₃ treatment via combined analysis of gene expression, metabolic pathways and protein profiles. The modulation of ROS scavenging, osmotic homeostasis, and reprogramming of multiple metabolic pathways jointly played important roles in the S. corniculata response to salt and alkali stress, which provides new insight into the fitness mechanism of S. corniculata under adverse conditions.